Anchoring of tensioned cables in concrete constructions



Jan. 20, 1942. E. FREYSSINET ANCHORING OF TENSIONED CABLES IN CONCRETECONSTRUCTIONS Filed Feb. 1, 1941 Patented Jan. 20, 1942 ANCHORING OFTENSIONED CABLES IN CONCRETE CONSTRUCTIONS Eugene Freyssinet,Neuilly-sur-Seinc, France Application February 1,1941, Serial No.377,040 In France August 26, 1939 14 Claims.

I have already shown that the characteristics of various constructionsof this nature can be considerably improved by utilizing reenforcementsof steel having a high elastic limit subjected initially to strongtensile stresses which produce in the construction a set of preliminarystresses opposite in direction to those produced by the application ofloads (see U. S. Patent 2,080,074). The tensile strain may be impartedto the reenforcements before or afterthe setting of the concrete. In thelatter case, the reenforcements, composed of cables or bundles of highresistance, drawn out, steel wires,'are laid out in hollows left in theconcrete to this effect. These reenforcements,- which in this case, donot adhere to the concrete, are subjected to tensile stresses when usingthe hardened concrete as fulcrum for the tensioning devices.

A difliculty of this well known process, lies in the anchoring membersto be established on the Wires or cables under tension to preventsliding of the latter in the concrete.

To this effect, steel or concrete gripping members have already beendevised comparable to those used in the anchorage of the cables ofsuspension bridges and consequently exterior to the construction, thetension in the reenforcements being obtained by the use of jacks actingbetween these gripping members and the main part of the construction.These devices are expensive and the space they occupy prevents 'anextension. of their application to current con structions such asbridges or buildings.

My present invention has for its object to provide grippingmemberswhich, among other advantages, eliminate all projections exteriorto constructions of concrete or other materials subjected to preliminarycompressionstresses.

This process is'particularly applicable to constructions of concretewhen the tensile strain is imparted to the reenforcements after thesetting of the concrete, although it can also be utilized inconstructions other than concrete or in concrete constructions when thereenforcements are strained before the setting of the concrete.

In what follows, only the application of the process to concrete will beconsidered for the terial of the construction-t0 be subjected topreliminary stresses. The said member is provided with a cavity throughwhich the cable passes; the elemental wires or strands of this cable arespread out against the inner surface of the cavity, they are then putunder tension and, while they are so strained, they are pressed againstthe inner surface of the cavity with sumcient compressive force toproduce between said wires or strands and said surface a frictioncapable of counterbalancing the tension of the wires and thus to preventthe sliding of the tensioned wires in the concrete.

It is preferable to increase this compressive force to such a value thatit will produce'an elastic expansion of the wall of the cavity whichcondition will contribute to holding the wires fast.

The compression of the wires of the cable against the inner surface ofthe cavity can be obtained by means of metallic wedges or pegs driven inby a jack or by a hammer in the axis of the cable or between its wires,or by hydraulic jacks composed of a sort of malleable bag which can beinflated by the injection under pressure, in the liquid state, of asubstance capable of ultimate hardening such as synthetic resin.

One method of obtaining this condition consists in embedding thewires-.of the cable, against the inner surface of the cavity, whilethese wires are under tension, in a substance adhering to the wires andcapable of hardening, such as cement mortar to which it is profitable tomix silicious or aluminous sand and asbestos to increase the friction ofthe wires on the mortar. Then a strong compression of this mortaragainst the inner surface of the cavity is obtained by introducing intothe latter a suitable instrument. It

is advisable to line the inside surface of the cavity with a metalliclining and to put grease or an equivalent lubricant between the saidlining and the inner wall of the cavity inorder to reduce the friction.The cavity having the shape of a truncated cone, and the compression ofthe mortar being obtained by a male cone wedged in the axis of thecable, the'slipping, which tends to occur on the greased surface as aresult of the tension of the wires when tensioning apparatus isreleased, compresses still more the mortar and increases the anchorageof the wires.

The body containing the cavity of which the cable is held fast may beconstructed in reenforced concrete; in this case, the cavity where thecable is held fast will be hollowed out in the concrete itself and theinner surface of this cavity will be made capable of resisting thestresses of expansion by embedding in the concrete a steel reenforcementwhich encircles the cavity and is put in place before the concrete ispoured. This reenforcement may consist of a steel tube or of helicoidturns of steel wire, having preferably a high elastic limit orof acombination of such'a tube and turns of wire wound about it. v

In order to obtain an eifective transfer to the concrete of the strainsborne by the said reenforcement, it is possible to provide a secondtransverse reenforcement of the concrete and composed either of a secondcoil formed by a steel wire, helically wound and located at a certaindistance from the first one, or of rectangular reenforcementsperpendicular to the axis of the cable.

My invention also comprises the apparatus necessary to the practicalrealization of my process.

The appended drawing, purely illustrative, and not inclusive of allcases, represents various embodiments of my invention.

Figure 1 represents a transverse sectional view \Figure 4 is the frontview of a group of 2 wires and of an anchoring jaw on the jack.

Figure 5 is part of the front view of a slotted washer perpendicular tothe cable, the function of which is to separate the wires prior to theirtensioning.

In the practical example represented on Figures 1 to 5, the cable to betensioned and anchored comprises 2 layers of wires or strands 8 and!(having in all an even number of wires or strands, 32 for example)grouped around a core consisting of a long pitch helicoid spring I. Itis enclosed in a sheath composed of, for example (Fig. 1) two metallicsheets I, la, hooked one to the other by bending back their edges. Thissheath is put in place in the molds before the concrete of theconstruction is poured and serves to insulate the cable with regards tothe concrete in order to allow the elastic elongation of this cable atthe moment of tensioning, even if at this moment the concrete hasalready set and hardened.

As can be seen in Figure 3, the extremity of the cable penetrates into atruncated steel member 6 provided with a funnel-shaped cavity and hoopedby a hardened steel wire 3 having, for example, an elastic limitsuperior to 35 kg. per square millimeter. This part 6 and the coil 3 areembedded in the concrete 2 poured about them. A concrete tight junctionID, of tape for example. joins part 6 to sheath I to prevent theconcrete from penetrating into this part and into this sheath. At theend of the slab of concrete 2, there is a thick steel washer l2 thepurpose of which is to sustain the thrust of the tensioning jack and totransmit it to the concrete.

A slotted plate I3 a portion of which figures in siderably reenforcedlongitudinally is poured between a tube I! (the axis of which lies alongthat of the cable) and a conical casing of smooth and polished sheetsteel I8 enclosed in another casing l9 made of brass or tin plate. Aconic casing 20, also made of tin plate or brass, is inserted in member6, the surface of contact between member 8 an dcasing 20 beinglubricated so as to obtain a coefficient offriction of the order of 5 to10%. The piston 2| of a jack bears upon washer l2 and is provided withsixteen radial slots 2la which allow the wires of the cable to pass bypairs. This piston 2| contains an auxiliary piston 22 bearing upon conel8. Cylinder 23 of piston 2| has sixteen anchoring jaws 24 each one ofthese receiving two-wires of the cable which are separated by anintermediate wedge 21 (a front view of which is seen on Figure 4 betweenthe projections 28 and 29 which make up the anchoring laws).

The concrete slab 2 comprises moreover a number of turns 25 of ordinarysoft steel and a set of reenforcements 26 parallel to the cable.

The cable with its sheath having been correctly placed in the molds ofthe construction, the extremity of the cable having been passed throughcasing 20 which lines the inside of the hollow member 6, and finally thecoils 3 and 25 being in place, the concrete 2 is poured.

Once the concrete has hardened, the wires of the cables are separated bypairs and made to pass between washers l2 and i4 and in the slots ofplate I3, which fastened by bolts, holds in place the whole fixture.

Then, cone IS with its tube l1, its casing I8 and the second casing 19,are inserted into the central hollow of the cable, and fresh plasticmortar containing silicious sand or asbestos is packed into the intervalbetween casing l9 and casing 20. By means of the jack, the piston 2i ofwhich bears, through the medium of washer l2, on the concretesurrounding tube 6, the steel wires, held fast by pairs on the anchoringjaws 24 of the cylinder of this jack, are tensioned; the

. cylinder drawing away from the concrete when pressure is admitted intothe cylinder.

Then, the wires being kept under tension, piston 22 is made to drive incone IS, the metallic casing !3 then sliding on casing H! with amoderate friction of metal against metal. The cone produces an intensecompression of the mortar between casings i9 and 20 of the order of 400kg. per square centimeter in the particular example which the drawingillustrates.

' The mortar loses its excess 'water and becomes very hard, and a highvalue is obtained for the friction angle of the mortar on itself and onthe steel; at the same time, the mortar takes up a minimum volume.

When the pressure in the cylinder of the jack is released the tension inthe steel is entirely transferred to that part of the device composed ofcone l6, casings i8 and I9, and the mortar between l9 and 20 and casing20. This ensemble attracted by the tension then slides along thelubrified surface between tube 6 and casing 20, which condition producesa very strong gripping eifect of the mortar on the wires of anapproximate value of 800 to 1000 kg. per square centimeter. Thisgripping effect obtains a perfect bond between the mortar and the wires,the angle of friction between surfaces 6 and 20, increased by the slopeof the cone, being inferior to the inner friction of the wires on themortar.

The elastic swelling of tube 6 and of coil 3 comtween the wires andtheir sheath. A warm substance, easy to melt, such as bitumen or aresin, may be injected after having heated the interior of the cables byinjecting hot air or steam,

The function of the core composed of the helicoid spring I is to ensurethe possibility of this injection and the regularity of the cable itselfin the curves which it assumes.

My invention is not confined only to the case where the tensioningoccurs after hardening of the concrete. The concrete in which thereenforcements under tension are embedded can be poured after thetensioning of these reenforcements, on condition that supports for thean chorage heads are available during this tensioning; these supportscan be on the molds or on a portion of the concrete slab in which thereenforcements are not embedded and which is poured in advance, andwhich has already hardened at the time of the tensioning.

It is obvious that the embodiments which have just been describedconstitute only examples and that these can'be departed from withoutaffecting the scope of the invention.

For example, in the case when the reenforcements are tensioned after thesetting and hardstruction at the place where the said extremityislocated. the setting of the group of wires in this cavity, thespreading out of the wires against the inner surface of the said cavity,the tension- 'ing'of thesewires by an external force, the embedding ofthe tensioned wires in a pasty substance adhering to the wires andcapable of hardening, then the compressing of this substance, after itshardening, against the inner surface of the cavity, after which the saidexternal force is removed.

3. The process for anchoring the extremity of a group of steel wiresconstituting a reenforcement of a concrete construction or the likewhich ening of the concrete, the sheath insulating the reenforcementwith respect to the concrete may consist merely of a plain coating of agreasy substance or of a plastic one of low melting point, basicallycomposed of bitumen, pitch or rubber and which is applied to the wiresand protected if need be by swathing with paper or other fibres. Thisplastic substance of low melting point may be softened at the true ofthe tensioning by means, for example, of an electric current sentthroughout the reenforcements.

My invention is applicableto the case where the elements of the cable tobe tensioned are constituted by strands or groups of wires in lieu ofsingle wires. The word "wire in the following claims is intended tocover as well as a single wire properly speaking a strand or a group ofwires.

What I claim is:

l. The process for anchoring the extremity of agroup of steel wiresconstituting a reenforcement of a concrete construction or the likewhich comprises: the provision of a cavity in the construction at theplace where the said extremity is located, the setting of the group ofwires in this cavity, the spreading out of the wires against the innersurface of the said cavity, the tensioning of these wires by an externalforce, then the compressing of the tensioned wires against the surfaceof the said cavity by a compression adapted to produce between the wiresand the said surface a friction capable of withstanding the tension ofthe wires, after which the said external force is removed.

2. The process for anchoring the extremity of a group of steel wiresconstituting a reenforcement of a concrete construction or the likewhich comprises: the provision of a cavity in the concomprises: theprovision of a funnel-shaped cavity, in the construction, at the placetherein where the extremity of the group is located. the saidfunnel-shaped cavity having its larger opening facing the saidextremity, the setting of the group of wires in this cavity, thespreading out of the wires against the inner surface of this cavity, thetensioning of these wires, by an external force, then the compressing ofthese tensioned wires against the surface of the said cavity by acompression adapted to produce an elastic deformation of the wall of thecavity, after which the said external force is removed.

4. The process for anchoring the extremity of a group of steel wiresconstituting a reenforcement of a concrete construction which comprises:the provision of a funnel-shaped cavity,

in the concrete at the place therein where the said extremity islocated, the said funnel-shaped cavity having its larger opening facingthe said extremity, the reenforcing of vthe wall of this cavity by meansof a reenforcement embedded in the concrete about this cavity, thesetting of the group of wires in this cavity, the spreading out of thewires against the surface of the said cavity, the tensioning of thesewires by an external force, then the compressing of the tensioned wiresagainst the surface of the said cavity by a compression adapted toproduce an elastic expansion of the said embeddedsteel reenforcement,after which the-said external force is removed.

5. The process for anchoring the extremity of a group of steel wiresconstituting a reenforcement of a concrete construction which comprises:the provision of a funnel-shaped cavity, in the concrete at theplacetherein where the said extremity is located, the said funnel-shapedcavity having its larger opening facing the said extremity, thereenforcing of the surface of this cavity by means of steel embedded inthe concrete about this cavity, the setting of the group of wires inthis cavity, the spreading out of the Wires against the surface of thesaid cavity, the tensioning of these wires by an external force, thenthe compressing of the tensioned wires against the surface of thiscavity by means of at least one wedge inserted in the cavity with aforce adapted to produce an elastic expansion.

of the said embedded steel after which the said external tensioned forceis removed.

6. The process for anchoring the extremity of a group of wiresconstituting a reenforcement of a concrete construction which comprises:the provision of a funnel-shaped cavity in the concreteat the placewherein the said extremity is located, the said funnel-shaped cavity'having its larger opening facing the said extremity, the reenforcing ofthe surface of this cavity by means of a steel coil embedded in theconcrete about this cavity, the setting of a hollow funnel-shaped partin the interior of the said cavity, the provision of a lubricant betweenthe said part and the surface of the cavity, the setting of the groupmortar after its hardening, against the surface of the cavity, by meansof at least one wedge inserted in the said mortar with a force adaptedto produce an elastic expansion of the said coil after which the saidexternal force is removed.

7. A device for anchoring a group of tensioned wires constituting areenforcement of a concrete construction, the said device comprising incombination: a concrete part in which is provided ;a funnel-shapedcavity, the inner surface of which surrounds the extremity of the wires,the

said cavity having its larger opening facing the exteriorof theconstruction, asteel reenforcement embedded in the concrete about thesaid cavity and means in the said cavity adapted to compress thewiresagainst the surface of the said cavity whereby the steelconstituting the said embedded reenforcement is elastically tensioned.

8. The process of simultaneous tensioning and anchoring of a group ofsteel wires constituting a reenforcement of a concrete constructionwhich comprises the setting of a hollow, funnel-shaped, steel shell overthe extremity of the said group, the larger opening of the said hollowshell facing the exterior of the construction, the pouring of theconcrete about the said hollow shell, the spreading out of. the wiresagainst the inner surface of the said shell, the setting of at least oneWedge in the said shell, the tensioning of the wires after the hardeningof the concrete, by applying an external tension to the extremity whichprojects out from the concrete, using the said concrete as fulcrum, thenwhile maintaining this external-tension, the driving in of the wedgetowards the smaller opening of the said hollow shell by a force adaptedto produce an expansion of the said shell, after which the said externaltension is removed.

9. In a concrete or similar structure, in combination, a mass ofconcrete or the like, a longitudinally tensioned reinforcement passingtherethrough and comprising an elongated member having a divided endportion comprising a plurality of strands, the mass of concrete adjacentone surface being provided with a cavity surrounding the stranded end ofsaid reinforcement, sa"d end being flared by the spreading out of thestrands and embedded in an adherent plastic material which preventsrelative movement between the strands and which forms therewith a hollowbody in contact with the walls of the cavity, and means exertingpressure in a direction generall radially of the axis of the member andforcing saidbody firmly against the walls of said cavity.

10. In a concrete or similar structure, in combination, a mass ofconcrete or the like, a longitudinally tensioned reinforcement passingtherethrough ,and comprising an elongated member having a divided endportion comprising a plurality of strands, the mass of concrete adjacentone surface being provided with a conical outwardly flared cavitysurrounding the stranded end of said reinforcement, said end beingflared by the spreading out of the strands and embedded in an adherentplastic material which prevents relative movement between the strandsand which forms therewith a hollow frusto-conical body inanti-frictional contact with the walls of the cavity, and a conicalwedge exerting pressure in a di-' rection generally radially of the axisof the member, disposed in anti-frictional contact with said body andforcing said body firmly against the walls of said cavity.

11. In a concrete or similar structure, in combination, a mass ofconcrete or the like, a longitudinally tensioned reinforcement passingthereth'rough and comprising an elongated member having a divided endportion comprising a plurality of strands, the mass of concrete adjacentone surface being provided with a conical outwardly flared cavitysurrounding the stranded end of said reinforcement, reinforcement meansfor the wall of said cavity, said stranded end being flared by thespreading out of the strands and" embedded in an adherent plasticmaterial which prevents relative movement between the strands and whichforms therewith a hollow frusto-conical body in contact with the wallsof the cavity, and means exerting pressure in a direction generallyradially of the axisof the member and forcing said body firmly againstthe walls of said cavity.

12. In a concrete or similar structure, in combination, a mass ofconcrete or the like, a longitudinally tensioned reinforcement passingtherethrough and comprising an elongated member at least an end portionof which is divided into a plurality of strands, the mass of concreteady'acent said end portion of the reinforcement being provided with aconical outwardly flared cavity surrounding said end portion and with ametallic reinforcement embedded in said concrete near the wall of saidcavity, said end-portion being flared by the spreading out of thestrands, means disposed between the spread strands and which formstherewith a substantially rigid frusto-conical body, said body beingpressed firmly against the wall of the cavity and tending to produce anelastic expansion thereof.

13. In a concrete or similar structure, in combination, a mass ofconcrete or the like, a longiiudinally tensioned reinforcement passingtherethrough and comprising an elongated member having a divided endportion comprising a plurality of strands, the mass of concrete adjacentone surface being provided with a conical outwardly flared cavitysurrounding the stranded end of said reinforcement, said end beingflared by the spreading out of the strands and embedded in an adherentplastic material which prevents relative movement between the strandsand which forms therewith a hollow frusto-conical body in contact withthe walls of the cavity. means exerting pressure in a directiongenerally radially of the axis of the member and forcing said bodyfirmly against the walls of said cavity. and metal sheathing coveringthe walls of said cavity and both the inner and outer walls of saidfrusto-conical body.

14. A device for anchoring a group of tensioned wires constituting areinforcement of a concrete construction. the said device comprisingincombination, a concrete part in which is provided a funnel-shapedcavity, the inner surface of which surrounds the extremity of the wires,the said cavity having its larger opening facing the exterior of theconstruction, a metallic reinforcement embedded in the concrete aboutthe said from, whereby the annular portion of the mass of concretebetween the two reinforcements is placed under greater compression thanthe rest of the concrete part when said first named rein- 5 forcement istensioned.

EUGENE FREYSSINET.

